Polyethylene terephthalate (“PET”) is used extensively in packaging applications, in particular as beverage containers. In these applications, it is important that the PET have a relatively high molecular weight, generally expressed as inherent viscosity (“IhV”) or intrinsic viscosity (“It.V.”), and low amounts of acetaldehyde.
There are two types of acetaldehyde (AA) to be concerned about. The first is residual or free AA contained in the pellets or particles sent to preform molders. The second type of AA is preform AA or the AA generated when the PET pellets are melt processed to make bottle preforms. AA precursors in the pellets can be converted to AA upon melting and give unacceptable levels of AA in the preforms. Melt processing also forms more AA precursors, which can liberate AA. Acetaldehyde has a noticeable taste and can be detected by human taste buds at low levels. When the preforms are blown into bottles, unacceptably high AA levels are those that adversely impact the taste of the beverage contained in the said bottles.
Relatively tasteless beverages such as water are particularly negatively impacted by the strong taste of AA. Many water bottle applications require lower levels of perform AA than carbonated soft drink (“CSD”) bottle applications. Converters who take polyester particles and make bottle preforms would like to have one resin that could be used to make preforms for both water and CSD applications. This would simplify the materials handling process at the converter by allowing for one feed silo or one type of feed silo, one product storage area or one type of product storage area etc. Most resins sold into water bottle markets have a lower It.V. than those resins sold into CSD markets. A dual use resin would have to a high enough It.V. for CSD applications and a low enough AA generation rate upon melting for water bottle applications.
In order to use one resin, some converters are adding AA scavengers to CSD resins to get acceptable perform AA for the water market. AA scavengers add significant cost to the container and often negatively impact the color of the container by making it either more yellow or darker as compared to an analogous container without AA scavenger added.
The conventional PET production process begins with esterification of predominantly terephthalic acid and ethylene glycol, or ester exchange of predominantly dimethyl terephthalate and ethylene glycol. The esterification need not be catalyzed. Typical ester exchange catalysts, which may be used separately or in combination, include titanium alkoxides, tin (II) or (IV) esters, zinc, manganese or magnesium acetates or benzoates and/or other such catalyst materials that are well known to those skilled in the art. The resulting mixture is then subjected to polycondensation in the melt at elevated temperature, for example 285° C., in the presence of a suitable catalyst. Compounds of Sn, Sb, Ge, Ti, or others have been used as polycondensation catalysts.
Following melt phase polycondensation, which generally achieves an inherent viscosity in the range of 0.5 to 0.65, the polyester is extruded, cooled, and cut into granules, which are then subjected to a crystallization process wherein at least the exterior of the granules becomes crystalline. This crystallinity is necessary to prevent sintering and agglomeration in a subsequent solid state polymerization. Crystallization and annealing take place in a fluidized bed at temperatures of, for example 160-220° C., for several hours, as discussed by WO 02/18472 A2, and U.S. Pat. Nos. 4,161,571; 5,090,134; 5,114,570; and 5,410,984.
Solid state polymerization or “solid stating” takes place in a fluidized bed over a period of from 10 to 20 hours, at a temperature which is preferably in the range of 180° C. to a temperature which is lower than the crystalline melt temperature by at least 10° C. Volatiles are removed in vacuo or by a flow of inert gas (e.g., nitrogen), or at lower temperatures, e.g. 180° C. or lower, by means of a flow of air. A variation in this process is disclosed in U.S. Pat. No. 5,393,871 where nitrogen containing water vapor is flowed through the solid stater.
Solid stating has the advantage that relatively high inherent viscosities can be achieved. It has the further advantage that acetaldehyde content of the polymer is lowered substantially by the removal of acetaldehyde by volatilization. Solid stating has the considerable disadvantages of high energy usage and long processing time. Finally, solid state polymerization causes the pellets to develop shell-to-core molecular weight gradients, which results in a loss in inherent viscosity during the molding of articles that is theorized to be due to re-equilibration in the melt.
It would be desirable to eliminate solid stating, but to do so would require more extended melt-phase polycondensation. In the absence of solid stating, removal of acetaldehyde present at the end of the melt phase polycondensation needs to be addressed. The situation is further complicated by the presence of acetaldehyde precursors which may later generate acetaldehyde, i.e., during injection molding of PET bottle preforms. Without solid stating, acetaldehyde precursors may remain at the concentration present after the melt-phase polycondensation.
When antimony catalysts are used for polycondensation, phosphorous compounds have been added to assist in lowering acetaldehyde and acetaldehyde precursors. However, antimony is not the most active catalyst, and deactivation of antimony with phosphorus compounds, if not performed carefully, may generate haze in the product. Titanium compounds are known to be much more active polycondensation catalysts, and can reduce the polycondensation time significantly. However, titanium compounds, when employed in PET production, often produce polymers with higher residual acetaldehyde, and can also result in greater generation of acetaldehyde downstream from polymer production per se, for example during the molding of preforms. Titanium catalysts also impart a distinct yellow cast to the product as well.
U.S. Pat. No. 5,656,716 discloses use of high surface area titanium catalysts followed by addition of triphenyl phosphate. Without the triphenyl phosphate, a high inherent viscosity but distinctly yellow product was obtained, while with triphenyl phosphate, less colored products are obtained, but only at a low inherent viscosity, thus requiring solid stating of these products with its disadvantages.
In WO 02/079310 A2, polyesters are stabilized against generation of aldehydes through addition of one of a diverse population of stabilizers, including sterically hindered amines such as Tinuvin® 123 or Tinuvin® 622 during initial esterification or transesterification. However, no salts of phosphorus-containing acids with these stabilizers are disclosed, nor is their addition late in a melt-phase polycondensation process.
In U.S. published application 2002/0198297 A1, nitrogenous stabilizers selected from hydroxylamines, substituted hydroxylamines, nitrones, and amine oxides are employed to scavenge acetaldehyde generated during extrusion of polyesters or polyamides. No salts made from these nitrogenous-stabilizers with phosphorus-containing acids are disclosed, nor is addition late in the melt-phase polycondensation stage of polyester production.
In World published application 2004/074365 A1, salts are made of hindered amine light stabilizers (HALS) derivatives and organophosphorus acids. Addition of amine salts to polyesters during the melt-phase manufacturing is not disclosed, nor is reduction of acetaldehyde.
In copending U.S. application Ser. Nos. 10/639,712; 10/382,103; 10/772,121; and 10/393,475, the disclosures of which are each incorporated herein fully by reference, phosphorus-containing acid salts of various amines and hindered amines are added during extrusion and disclosed as useful in maintaining polycarbonate molecular weight during extrusion of polyester/polycarbonate blends, while reducing color as well. Addition of amine salts to polyesters during the melt-phase manufacturing is not disclosed, nor is reduction of acetaldehyde. It would be desirable to be able to produce PET and other polyesters with an inherent viscosity suitable for production of food and beverage containers, without the necessity for solid stating, which exhibit lower content of acetaldehyde, and/or which generate reduced levels of acetaldehyde during further processing. It would further be desirable to produce PET in shorter reaction time, due to a more active catalyst than antimony, while maintaining or improving upon the AA properties of the product, with or without solid state polymerization.